<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

    <ul>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
    </ul>
    <button>追加</button>

</body>
<script src="../js/jquery.min.js"></script>
<script>
    $(() => {
        $('ul').on('click', 'li', function() {
            $(this).css('backgroundColor', 'red');
        })
        $('button').on('click', () => {
            $('ul').append($(document.createElement('li')));

        })
    })
</script>

</html>